This invention relates to a Code Division Multiple Access (CDMA) communication system and more particularly to a CDMA system integrated with a remote microcell communication system.
Microcells in a cellular communication system allow coverage and additional capacity in an area that is not reachable by a base station cellsite, such as a CDMA cellsite. The CDMA system has up to six faces. Three are physical faces, which are the three physical faces of the system is antenna. There are also three virtual faces that behave as a physical face but are not a physical part of the antenna of the CDMA system.
Each virtual face can be a remote microcell, and more than one remote microcell can be integrated with each face. A remote microcell is installed in the area of desired coverage and is then connected back to the base station cellsite through appropriate channels, such as optical fiber.
Remote microcell arrangements, however, experience time delays in the signal caused by the excessive amount of time required for the CDMA signal to travel from the base station cellsite to the remote microcell, and then to travel through the internal circuitry of the microcell itself. CDMA is a synchronized system, working in conjunction with the Global Positioning Satellite (GPS) system, and some microcell systems do not have re-synchronizing capabilities. Therefore, the time delayed signal emitted by the microcell can also be out of synch with the rest of the CDMA system causing communication problems.
The delayed signal is often compensated for by setting an extremely large search window size parameter to allow a mobile device, i.e. a handset, to access the system. The mobile device will accept a late signal and has the capability to synthesize the late signal. However, this takes up processing power. In addition, a wider search window size takes more time to scan and therefore, adds more time delay in the system making it less reliable. It is desirable to have a narrow search window for faster, reliable service.
The excessive delay caused during a roundtrip of the signal can also result in call setup failures. In general, a predetermined sector size that is broadcast to the mobile device establishes the area to be served. Radio waves take a set amount of time to travel through the air and return. A limit is set for the amount of time given for a signal to travel to the mobile device and broadcast back out, which effectively circumscribes an active communication circle around the mobile device. Because of signal delay in the optic fiber, the CDMA system can perceive the signal delay as an indication that the mobile device is much farther away from the microcell than it actually is and the call will not be allowed on the system, thus resulting in call setup failures. It is desirable to have a large sector size, but the time delay associated with a large sector size is undesirable.
Prior art devices have compensated for the delay between CDMA and a remote microcell by inducing delay in order to provide the appearance of a synchronized system. In the prior art, the delay is induced to allow greater than one microsecond of difference between the two signals so that a receiver can provide multipath signals. The system appears to be synchronized by providing multiple paths for signals to travel. However, this approach does not address the problem of extremely large search window sizes and call setup failures.
It is an object of the method of the present invention to integrate a microcell system that does not have a re-synchronizing device, with a CDMA system to eliminate the above mentioned problems caused by signal delay associated with remote microcells and CDMA base stations.
It is another object of the present invention to provide a method for integrating the microcell system with the CDMA system by advancing the signal in order to compensate for time delay generated as a result of the signal leaving the CDMA cellsite, traveling to the remote microcell and returning to the CDMA cellsite.
It is a further object of the present invention to calculate suitable values for padding the sector size to eliminate call setup failures due to the signal delay between the remote and the cellsite.
The present invention is a method of integrating a microcell system with a CDMA system. The microcell system does not have re-synchronizing capabilities, such as an ADC Kentrox(trademark) system, while the CDMA system has signal advancing capabilities, such as a Lucent(trademark) system.
The present invention provides a method that determines the amount to advance the CDMA signal so that when it reaches the remote microcell, the time delay has been compensated for and the signal is synchronized with the CDMA system. The method also provides for nullifying the round trip time delay allowing call setups to occur normally.
The objects and features will become more apparent to one skilled in the art from the following detailed description taken together with the accompanying drawings and the claims appended hereto.